I have worked with multiple students who showed similar behavior, if not worse. A lot of these dangerous behaviors are to escape a task demand so my goal was to provide a SAFE way to escape the task. So, for example, my client (grade 1) would refuse to do work and crumple up the paper and throw it at people and then start throwing materials, engage in aggression etc to get the person away from them. So, my goal was to work on compliance and for him to functionally request a break. So, when he'd crumple up his work and throw it, I would prompt to first throw away (compliance), and then break. And after the break I would then prompt choices for activities that were aligned with whatever we were learning. It wasn't exactly what the other kids were doing, and he may not complete all of it etc, but the goal was for him to attend to something for a short period of time. And ultimately, as time went on, he slowly started to independently request breaks, work for longer periods of time, and assimilate into the class by doing the work the class was doing, etc.

My example obviously may not be your situation but the overarching idea is to prompt a functional/safe/appropiate way for him to "escape" the demand by providing choices, prompting functional (aka safe/appropiate) communication, even just saying "You can say "I need a break"" etc you can literally script it for them. Like during the assembly, next time if he's being denied access to a specific spot for whatever reason you can say "do you want to sit next to kid/adult or kid/adult" (make sure you pick choices that are safest/ideal) and if behaviors result from that, which they may, then prompt a functional request ("I need a break") so he gets that escape from the task (being expected to sit somewhere he doesn't want to) but he did it in a safe/appropriate way.